Earth Waves

Earth Waves

Nancy Marie Brown

December 01, 2004

No other Hollywood movie has waves like these.

John Hammer flicks on his video: it's blurry, black-and-white, basically low-budget. A thick black line rushes in at an angle, splits, bends, scatters, and leaves a wake of wavy circles. It's a look at the earth underneath Los Angeles, as a far-off earthquake undulates its way through.

"There are a couple of things you can see here," says Hammer, a geophysics graduate student at Penn State. He rewinds and replays the video frame by frame. "Here's a reflection of the wave off the bottom of the Basin . . . You can see the wave sort of shorten up here—the velocity is slower inside the Basin because the material the wave travels through is different. Below the Basin it's solid coherent rock. Inside the Basin, it's sand and gravel. . . ."

The Los Angeles Basin is a very well-studied slice of the planet. Oil companies and academics both have drilled expensive boreholes hundreds of feet down, learning that the city sits on "an inverse mountain," according to Hammer's adviser, geophysicist Chuck Langston. "Buckling of the Earth's crust formed a hole next to the San Gabriel mountains," Langston explains, "which has been filled in."

The complexity of the Basin makes for very noisy seismograms at the earthquake-monitoring station of the University of Southern California—even those of far-away earthquakes, say, from the Philippines or Peru.

"We'd expect to see a nice simple wave form," explains Hammer. "When waves come from that far away, we expect them to come straight up under the station." Instead, USC's seismograms are a near-unreadable clutter of squiggles and bumps. "Most people just drop USC" from their data set, Hammer says, when trying to pinpoint the epicenter of a distant earthquake.

"They've known for years it's the Basin that's making it noisy, but no one's quantified what the Basin is doing, how it's interacting with the wave. My aim is to explain exactly why this thing becomes so complicated."

Working from the known structure of the Basin and the records of a far-off earthquake, Hammer simulates the earthquake wave running up through a model of the Basin. He then tries to match the reflections and diffractions on his computer video with the actual peaks and troughs on the seismogram.

"If we understood what the Basin was doing to the records, we could remove the noise and make the data more usable," Hammer explains.

"And, if this works out in the L.A. Basin, we may be able to run the model backwards in areas where we don't know so much about the geology." That is, rather than using simulations of the earth's structure to create readable seismograms, use an observed seismogram to understand the structure of the earth—a much cheaper and simpler process than digging deep holes."Once we've shown with something that's well-constrained, like the L. A. Basin, that this works, we'll look at a couple of other earthquake-monitoring stations in the region, and get an idea what structures are under them.

"It's nice for hazard assessment as well. It makes it easier to understand events like the recent Northridge earthquake, when you can run models to see how the structure of the earth is interfering with the earthquake."